This invention relates to blow molding and, more particularly, relates to an improved method for blow molding thermoplastic articles, such as bottles, in which the blow molding fluid pressure is monitored as an inspection tool.
Thermoplastic bottles are replacing glass containers in applications where breakage resistance is an important factor. For instance, thermoplastic bottles are now widely used for packaging household products, and certain polymers, such as poly(ethylene terephthalate), hereinafter PET, are being proposed for use in carbonated soft drink containers.
Thermoplastic bottles can be commercially manufactured by inflating a hollow preform against a mold cavity employing either injection blow molding or reheat blow molding techniques. The quantity of polymer per bottle can be minimized, for most polymers, by conducting the process at the appropriate temperature to impart orientation in the bottle sidewall, thereby improving bottle strength. Mechanical stretching is often employed, either concurrent with or prior to the inflation step, to obtain optimum orientation.
The behavior of thermoplastic materials, during inflation, is sensitive to variations in blow-molding pressure and preform temperature, geometry, and molecular weight. Unless these variables are within prescribed limits, the resulting bottle will be defective. For instance, an improperly inflated PET bottle may have a milky-white appearance due to polymer crystallization, unacceptable wall thickness variations, and in extreme situations, folds or holes in the bottle walls.
Although conventional process controls minimize the production of defective bottles, it remains necessary to inspect the bottles because so many variables influence the process and process controls are not fail-safe. An automatic method for inspecting the bottle is especially desired to minimize the number of inspectors, to abort defective bottles prior to any subsequent manufacturing operation, and to provide an early alert if the process is not operating properly.
Polymer viscosity, preform configuration and temperature, and blow molding pressure all influence a blow molding operation and are selected to cause a thermoplastic preform to expand in a predetermined manner during inflation.
When making a biaxially oriented bottle, for instance, a preform temperature and/or geometry profile can be employed to progressively initiate inflation in the neck, sidewall, and base regions of the preform. Blowing pressures are employed which do not force the expanding preform into contact with the chilled mold until the desired level of orientation is reached since earlier contact will set the polymer, making further stretching difficult, if not impossible, without damage to the bottle walls. This procedure places additional polymer in the bottle base, which is less oriented than the bottle sidewall, thereby achieving desired, over-all strength with a minimum of polymer.